Q&A 3 that a change of net price of a PO is still possible after a GR and IR (even it does not make much sense if you have follow on documents), A new price determination is not possible

And in OSS note 456691 - FAQ: Price determination in purchasing Q&A 34 you can read that changing of conditions of type B delivery costs is not possible anymore after an invoice receipt (even after a cancellation of the same)

First you need to check, whether your condition type has the ability for manual intervention through t code M/06, if you can maintain that condition manually, than you need to delete that condition and fill it again, but it will re-trigger your release strategy!